High pressure piezoresistive transducer suitable for use in hostile environments and method for making the same

ABSTRACT

A pressure transducer including: a silicon substrate including: a first surface adapted for receiving a pressure applied thereto, an oppositely disposed second surface, and a flexing portion adapted to deflect when pressure is applied to the first surface; at least a first sensor formed on the second surface and adjacent to a center of the flexing portion, and adapted to measure the pressure applied to the first surface; at least a second gauge sensor formed on the second surface and adjacent to a periphery of the flexing portion, and adapted to measure the pressure applied to the first surface; a glass substrate secured to the second surface of the silicon wafer.

FIELD OF INVENTION

[0001] The present invention relates to pressure transducers and moreparticularly to an improved high pressure piezoresistive transducerwhich is suitable for use in hostile environments and a novel,advantageous method for making the same.

BACKGROUND OF INVENTION

[0002] Kulite Semiconductor Products, Inc., the assignee herein, haspreviously made and patented a method for fabricating high pressurepiezoresistive transducers using both longitudinal and transversepiezoresistive coefficients U.S. Pat. No. 5,702,619, entitled “Method ofFabricating a High-Pressure Piezoresistive Transducer”, filed Sep. 30,1996, and assigned to the assignee herein, the entire disclosure ofwhich is hereby incorporated by reference. Therein, a basic sensor isformed from a piece of single crystal silicon to which sensors aredielectrically bonded on one surface and the other surface of thesilicon is bonded to a glass support member. In those structures thepiezoresistive elements were formed on the surface of the transducerthat is directly exposed to the pressure media. Additionally, electricalcontacts and lead wires are also exposed to the media.

[0003] This structure is undesirable in some situations, where exposureof the piezoresistive elements, electrical contacts and lead wires tothe media shortens the life expectancy of the pressure transducer.Accordingly, it is an object of the present invention to provide a highpressure transducer less sensitive to the media.

SUMMARY OF INVENTION

[0004] A pressure transducer including: a silicon substrate including: afirst surface adapted for receiving a pressure applied thereto, anoppositely disposed second surface, and a flexing portion adapted todeflect when pressure is applied to the first surface; at least a firstsensor formed on the second surface and adjacent to a center of theflexing portion, and adapted to measure the pressure applied to thefirst surface; at least a second gauge sensor formed on the secondsurface and adjacent to a periphery of the flexing portion, and adaptedto measure the pressure applied to the first surface; a glass substratesecured to the second surface of the silicon wafer.

BRIEF DESCRIPTION OF THE FIGURES

[0005]FIG. 1 illustrates a diagram of the approximate shape of adiaphragm with the resistor placement according to the presentinvention.

[0006]FIG. 2 illustrates a top view of a glass support to which thediaphragm of FIG. 1 is mounted according to the present invention.

[0007]FIG. 3 illustrates a cross-section A-A of FIG. 2.

[0008]FIG. 4 illustrates the mounting of the diaphragm/support assemblyto a header according to the present invention.

[0009]FIG. 5 illustrates a cross-section of a diaphragm/support andheader according to the present invention.

[0010]FIG. 6 illustrates a perspective view of gauge placement accordingto the present invention.

[0011]FIG. 7 illustrates a side view of a sensor assembly as describedin U.S. Pat. No. 5,614,678.

[0012]FIG. 8 illustrates a stress diagram for the sensor of FIG. 7.

[0013]FIG. 9 illustrates a sensor assembly according to the presentinvention.

[0014]FIG. 10 illustrates a stress diagram for the sensor of FIG. 9.

DETAILED DESCRIPTION OF INVENTION

[0015] Referring now to the numerous figures, wherein like referencesrefer to like elements of the invention, FIG. 1 illustrates a diagram ofthe approximate shape of a diaphragm with the resistor placementaccording to the present invention.

[0016] According to the present invention, piezoresistive elements areplaced on a side of the silicon structure 10 isolated, or away, oropposite from a media. Preferably, two elements or gauges, 15, 20 arelocated near a center of flexing portion 25 of the silicon member 10,while two additional members or gauges 30, 35 are located just insidethe flexing portion 25 area.

[0017] Contact areas 40 on the silicon structure 10 are sealed to aglass support structure 45 in a non-flexing area (complement of flexingarea 25). Referring now also to FIGS. 2 and 3, holes 50 are provided inthe glass support structure 45 to access the various contact areas 40 ofthe silicon structure 10 associated with sensors 15, 20, 30 and 35.Additionally, a small depression 55 to allow the flexing area portion 25of the silicon structure 10 to deflect is provided. The sensor network(sensors 15, 20, 30 and 35) and contact areas 40 are preferablydielectrically isolated from the silicon structure 10 in the same manneras U.S. patent application Ser. No. 09/047,548, entitled “CompensatedOil-Filled Pressure Transducers” filed Mar. 25, 1998, the entiredisclosure of which is also incorporated by reference hereinto,including the seal of the glass to a rim structure and to the contactareas 40.

[0018] Referring now also to FIGS. 4 and 5, the apertures, or holes, 50in the glass structure 45 are preferably partially filled with ametallic frit 60 (and or an epoxy metal frit for example) and smallcopper balls 65 are inserted in the back areas of the apertures 50 (alsosee FIG. 9). The sensor-glass support structure (collectively 10 and 45)is then mounted to either a polymide structure 70 or ceramic structurewith plated through holes 75 into which the exposed portions of thecopper balls 65 will seat.

[0019] Contact can be made between the balls 65 and plated through holes75 with a solder or braze. If a polymide structure 70 is used, thesensor structure can be secured with an epoxy or like material, while ifa ceramic structure 70 is used the mounting may be made using a glasstype frit. However, both mounting surfaces contain lead outs, ormetallizations 90 to a series of holes 80, sized in such a way toconform to the position of pins 95 on a header 85 preferably securedutilizing tapered glass 100.

[0020] The composite structure (10, 45 and 70) is then mounted on theheader 85 allowing the interconnects and the composite structure (10, 45and 70) to be electrically connected to the pins of the header 85. Whenpressure is applied from the side of the silicon not containing thesensor network, i.e. opposite thereof, the central portion of thesilicon structure 10 deflects giving rise to a tensile surface strain inthe center of the flexing member 25, while the exterior portions of theflexing member 25 will be put in compression.

[0021] Referring now also to FIG. 6, methods of finite analysis wereused to elucidate the various stresses within the plane of the siliconstructure 10 (directions 105 and 110) and normal to it (direction 115).This analysis shows that the region of compressive surface stress in theflexing portion 25 where the sensor may be placed is very narrow. Thisis because the compressive normal stress in the center of the flexingregion 25 is zero, but rises to its largest value at the outer edge ofthe flexing member 25, and because of the negative sign of thetransverse gage factor in the <110> direction is negative. If the outergauge is in this region, the change in resistance will be positive[(−1)×(−1)] and there will be no output.

[0022] In general, each gauge sees three different stresses: alongitudinal stress in the plane of the diaphragm (direction 110), atransverse stress in the plane of the diaphragm (direction 105), and atransverse stress perpendicular to the diaphragm (direction 115). Thesestresses serve to change the resistivity of the gauge throughpiezoresistive effects. In general this change in resistivity can bebroken down into a change for each separate stress, namely:$\begin{matrix}{\frac{\Delta \quad R}{R} = {{\sigma_{x}\pi_{x}} + {\sigma_{y}\pi_{y}} + {\sigma_{z}\pi_{z}}}} & (1)\end{matrix}$

[0023] where σ is the stress is one of the three directions and π is thepiezoresistive coefficient in that same direction.

[0024] By appropriate choice of crystallographic orientation, oneskilled in the art can ensure the coefficient in the longitudinal inplane (110) and transverse out of plane (115) are equal in magnitude andopposite in sign, while the coefficient for the transverse in plane(105) is very close to 0. This leads for a final result for the changein resistance to be: $\begin{matrix}{\frac{\Delta \quad R}{R} = {{\sigma_{long}\left( \frac{\pi_{44}}{2} \right)} - {\sigma_{tran}\left( \frac{\pi_{44}}{2} \right)}}} & (2)\end{matrix}$

[0025] By finite element analysis one can compute the transverse andlongitudinal stresses that the gauges see and therefore choose thelocations which yield the maximum change is resistance for a given loadcondition.

[0026] Referring now also to FIGS. 7-10, therein is illustrated a not toscale drawing and a graph of the relevant stresses for both aconventional high pressure sensor (FIGS. 7-8) and the new leadless one(FIGS. 9-10). FIGS. 7 and 9 are for reference only and should be used toclarify FIGS. 8 and 10. FIGS. 8 and 10 illustrate the transverse andlongitudinal stresses in the appropriate part of the diaphragm. FIGS. 8and 10 also have marked the approximate locations for the placement ofthe gauges.

[0027] Referring first to FIG. 7, therein is illustrated a conventionalpressure transducer including supports 120, diaphragm 125 and gauges130.

[0028] It can be seen from FIGS. 8 and 10 that for each sensor there aretwo gauges which will see a negative change in resistance and two whichwill receive a positive change in resistance. By combining these fourgauges in a wheatstone bridge as set forth in U.S. Pat. No. 3,654,579,entitled “Electromechanical Transducers and Housings” filed May 11,1970, is assigned to the assignee hereof, also herein incorporated byreference, one can to achieve the desired change in voltage.

[0029] This new structure has a number of unanticipated advantages. Theposition of both the inner and outer gages was only learned bycomputation using finite element analysis and would be different foreach geometry of the sensor but the large difference in surface stressdistributed from the top to the bottom surface of the silicon was notanticipated. However, the use of the finite analysis still makespossible the fabrication of a miniature sensor.

[0030] By putting the sensing network on the side of the silicon awayfrom the media and using glass support structures with access holes toreach the contacts, it makes possible the construction of a “leadless”structure without fine gold wires and ball bonds as is illustrated inpending U.S. patent application Ser. No. 09/160,976 entitled“Hermetically Sealed Ultra High Temperature Silicon Carbide PressureTransducers and Method for Fabricating Same” filed Sep. 25, 1998. Italso makes possible higher temperature application of the device sincethe contact material in the apertures is sealed from any hightemperatures, hostile environment while still retaining all of theadvantages of the structure disclosed in pending U.S. patent applicationSer. No. 09/160,976 entitled “Hermetically Sealed Ultra High TemperatureSilicon Carbide Pressure Transducers and Method for Fabricating Same”filed Sep. 25, 1998. Additionally, the use of a separate mountingsurface for the sensor structure makes possible the use of a headerspecifically designed for high pressure while still employing aminiature sensor.

[0031] Having described the preferred embodiment of this invention, itis evident that other embodiments incorporating these concepts may beused. Accordingly, although the invention has been described andpictured in a preferred form with a certain degree of particularity, itis understood that the present disclosure of the preferred form has beenmade only by way of example and that numerous changes in the detail ofconstruction in combination and arrangement of parts may be made withoutdeparting from the spirit and scope of the invention as here and afterclaimed. It is intended that the patent shall cover by suitableexpression in the appended claims, the whatever features of patentablenovelty exist in the invention disclosed.

I claim:
 1. A pressure transducer comprising: a silicon substrateincluding: a first surface adapted for receiving a pressure appliedthereto, an oppositely disposed second surface, and a flexing portionadapted to deflect when pressure is applied to said first surface; atleast a first sensor formed on said second surface and adjacent to acenter of said flexing portion, and adapted to measure said pressureapplied to said first surface; at least a second sensor formed on saidsecond surface and adjacent to a periphery of said flexing portion, andadapted to measure said pressure applied to said first surface; a glasssubstrate secured to said second surface of said silicon wafer.
 2. Thetransducer of claim 1, further comprising at least a third sensor formedon said second surface and adjacent to said center of said flexingportion, and adapted to measure said pressure applied to said firstsurface.
 3. The transducer of claim 2, further comprising at least afourth sensor formed on said second surface and adjacent to saidperiphery of said flexing portion, and adapted to measure said pressureapplied to said first surface.
 4. The transducer of claim 3, whereinsaid first and third sensors are further adapted such that resistancesrespectively associated therewith decrease when said pressure to bemeasured is applied to said first surface of said silicon substrate. 5.The transducer of claim 4, wherein said second and fourth sensors arefurther adapted such that a resistance associated therewith increaseswhen said pressure to be measured is applied to said second surface ofsaid silicon substrate.
 6. The transducer of claim 5, wherein saidfirst, second, third and fourth sensors are formed within said flexingportion.
 7. The transducer of claim 1, further comprising: a pluralityof contact areas formed on said second surface of said silicon wafer; aplurality of apertures in said glass substrate, each corresponding toone of said plurality of contact areas; a plurality of conductive frits,each positioned within a corresponding one of said plurality ofapertures; and, a plurality of conductive balls each also positionedwithin a corresponding one of said plurality of apertures.
 8. Thetransducer of claim 7, further comprising: a header including aplurality of pins; and, a mounting board coupled between said header andsaid glass substrate and adapted to respectively electrically coupleeach of said balls to a corresponding one of said pins.
 9. Thetransducer of claim 8, further comprising a plurality of apertures insaid mounting board, each of said apertures in said mounting boardcorresponding to either one of said plurality of pins, or one of saidplurality of balls.
 10. The transducer of claim 9, wherein said mountingboard comprises a material selected from the group consisting of: apolymide and a ceramic.
 11. A method for making a pressure transducercomprising the steps of: providing a silicon substrate including a firstsurface, an oppositely disposed second surface, and a flexing portionadapted to deflect when said pressure to be measured is applied to saidsecond surface; forming on said first surface and adjacent to a centerof said flexing portion at least a first sensor adapted to measure apressure applied to said second surface; forming on said first surfaceand adjacent to a periphery of said flexing portion at least a secondsensor adapted to measure said pressure applied to said second surface;and, sealing a glass substrate including a plurality of apertures tosaid first surface of said silicon substrate.
 12. The method of claim11, further comprising the steps of: partially filling each of saidplurality of apertures with a metallic frit; inserting a conductive beadinto each of said plurality of apertures; and, securing said glasssubstrate to a header adapted to at least partially receive each of saidplurality of metallic beads.
 13. The method of claim 12, furthercomprising the steps of: coupling a plurality of pins to said header;and, respectively electrically coupling each of said balls to anassociated one of said plurality of pins.
 14. The method of claim 11,further comprising the step of forming on said first surface andadjacent to said center of said flexing portion at least a third sensoradapted to measure said pressure applied to said second surface.
 15. Themethod of claim 14, further comprising the step of forming on said firstsurface and adjacent to said periphery of said flexing portion at leasta fourth sensor adapted to measure said pressure applied to said secondsurface.
 16. The method of claim 15, wherein said step of sealing saidglass substrate to said first surface of said silicon substrate furthercomprises the step of isolating said at least first, second, third andfourth sensors from a media associated with said pressure to bemeasured.
 17. The method of claim 15, wherein said step of forming saidfirst, second, third and fourth sensors, further comprises the step offorming said first, second third and fourth sensors within said flexingarea.
 18. A method for forming a pressure transducer including a sensornetwork which is isolated from a media associated with a pressure to bemeasured, said method comprising the steps of: forming on a firstsurface of a substrate including an oppositely disposed second surfaceand a flexing portion adapted to deflect when a pressure to be measuredis applied to said second surface, and adjacent to a center of saidflexing portion at least a first gauge adapted to measure said pressureapplied to said second surface; forming on said first surface andadjacent to a periphery of said flexing portion at least a second gaugeadapted to measure said pressure applied to said second surface; and,sealing a glass substrate including a plurality of apertures to saidfirst surface of said substrate and over said at least first and secondgauges, thereby isolating said first and second gauges from said mediaassociated with said pressure to be measured.
 19. The method of claim 18wherein said step of forming said at least first gauge comprises thestep of forming said at least a first gauge in a first position selectedsuch that a resistance associated with said first gauge decreases whensaid pressure to be measured is applied to said second surface of saidsubstrate.
 20. The method of claim 19, wherein said step of forming saidat least second gauge comprises the step of forming said at least secondgauge in a second position selected such that a resistance associatedwith said second gauge increases when said pressure to be measured isapplied to said second surface of said substrate.